Calcium homeostasis in the cytosol of vertebrate and invertebrate muscle cells is essential for normal cellular activity. It is a complex process involving balancing calcium from extracellular sources that moves through calcium selective channels on the plasma membrane of the cell, and calcium in internal stores controlled by a calcium activated calcium release channel located in the membrane of the sarcoplasmic reticulum. Release of calcium from the sarcoplasmic reticulum plays a crucial role in excitation-contraction coupling in muscle tissue (Pessah et al., (1986) J Biol Chem 261:8643–8648) and the initiation and propagation of calcium signaling events.
The calcium activated calcium release channel internal to the cell is called the ryanodine receptor (Ryr) because of the interaction of the receptor with a natural insecticide ryanodine, an alkaloid from the Ryania speciosa. Ryanodine binding sites have been studied in an effort to understand the properties of the Ca2+ release channel in insects as a possible target for insecticide action (Lehmberg et al., (1994) Pesticide Biochem and Physiol 48: 145–152). However, no synthetic insecticides acting on the ryanodine receptor as a primary mode of action have been discovered until recently. Chemistry based on derivatives of anthranilamides with very potent activity on pest species activate this receptor in a manner leading to calcium release, thereby disrupting the calcium balance of the insect cell. The response of the organism as a result of this disruption is unrelieved muscle contraction, including cardiac, skeletal and pharyngeal muscles, leading to lethargy and cessation of feeding.
It is reported in insects that there is only one form of the receptor-ion channel. Studies in insect tissues indicate that the receptor has similar properties and size to its mammalian homologue (Denser et al., (1998) Pestic. Sci. 54:345–352). The ryanodine receptor has been most studied in mammals where there are three currently recognized isoforms (types 1, 2, and 3) which are subject to differential regulation and have different tissue distributions.
Full-length genomic DNA sequence (Takeshima et al. (1994) FEBS Lett., 337: 81–87) and cDNA sequence (Xu et al. (2000) Biophys J. 78: 1270–1281) of the Drosophila gene encoding a ryanodine receptor is known and available from public databases (NCBI Accession No. D17389), from other invertebrates, e.g. Caenorhabditis elegans (NCBI Accession No. D45899) and a small segment of the C-terminus of the ryanodine receptor from the tobacco budworm, a lepidopteran pest (Heliothis virescens; Puente et al.,(2000) Insect Biochem. Mol. Biol. 30: 335–347). The sequence from Drosophila has been cloned and expressed in Chinese Hamster Ovary (CHO) cells and the receptor-ion channel shown to be functional using electro-conductance techniques (Xu et al. (2000) Biophys J. 78: 1270–1281).
Crop destruction by pests such as insects results in a considerable economic loss and serious reduction in productivity. For example, lepidopteran pest species cause $500 MM of damage to various crop species annually. Homopteran pests account for a further $2000 MM. Recently, two areas of chemistry have shown good control of lepidopteran pests. From an analysis of intracellular calcium changes and the physiological response of the pest to the compounds the mode of action is by disruption of normal muscle function through the release and eventual depletion of internal calcium stores in the muscle and central nervous system. Thus, based on the response of cells expressing the ryanodine receptor either in natural or recombinant systems to various types of low molecular weight chemical structures, it is clear the receptor and potentially other attendant components that interact with it are important targets for pest control.